278 J. T. CUNNINGHAM. 



system which forms the mus. levator. The conclusion which 

 must be drawn is that the hyomandibular in the Rays is a 

 remnant of an arch anterior to and entirely distinct from the 

 hyoid arch, while in the Sharks the dorsal part of the hyoid 

 arch with its rays is fused with the hyomandibular. According 

 to Gegenbaur the hyomandibular in the Rays represents only 

 the mandibular process of the hyomandibular of the Sharks ; if 

 this were true there would be no rays dorsal to the venous 

 commissure in the Rays, whereas the fact is that these dorsal 

 rays exist, but the cartilage they belong to is separate from 

 the hyomandibular. Dohrn finally suggests that the upper 

 jaw is also an independent gill arch, and the mandible another, 

 but for the present leaves the further tracing of the transfor- 

 mations for more profound investigations. He concludes the 

 section on the hyoid arch in the Rays with the remark that he 

 is satisfied to dispel the illusion that we already know what we 

 want to ascertain. 



The spiracular cartilage is next taken in hand. Dohrn has 

 investigated its origin in Scyllium canicula and catulus, Pris- 

 tiurus, Mustelus, Raja and Torpedo. He found it always a 

 single cartilage, and states that there is no foundation for the 

 theory that it is either an enlarged single ray, or a combina- 

 tion of rays. It is probably a portion of an independent 

 arch, but what relation this arch bears to others it is at present 

 impossible to say. The adductor mandibulse is developed from 

 the whole of the walls of the mandibular head cavity, no 

 portion being separated off as an adductor; only one diflPeren- 

 tiation of a portion occurs, namely, the formation of the levator 

 maxillse superioris from the part lying nearest to the spiracle. 



When it has been postulated that the hyoid arch is really 

 double and contains two arches fused together, it becomes 

 necessary to inquire what has become of the cleft originally 

 existing between these two arches. Has the cleft disappeared 

 without leaving a trace, or has it merely undergone a meta- 

 morphosis ? Dohrn answers that the pair of clefts, i. e. the 

 endodermal parts of them, have united in the median ventral 

 line and formed the thyroid gland. Tliis organ arises in the 



